I have created a UDF to display "Success" (to keep this simple) every time one of the cells in the range D1:D4 changes.

However, 3 out of 4 of the cells contain real-time data that is refreshed every 1500 milli-secs and so every time the cells refresh the function triggers the MsgBox to display even though the values haven't changed. I only want the MsgBox to display if the values in those cells change.... can anyone help, I have spent the last 3 days trolling the net for a resolution but to no avail?

=Trigger(D1:D4)

Function Trigger (theParameter As Variant)
      MsgBox "Success"
End Function
Appreciation in advance.